Inhaled anthrax infection is a major bioterrorism threat today. Models that simulate this disease for the study of pathogenesis and treatment are needed. Anthrax infection begins as a local collection of alveolar spores, which then spread as invasive bacteria to the mediastinal structures. From there the infection is disseminated systemically by intravascular spread. Intravascular spread results in a increasing toxin release that contributes directly to death. Anthrax bacilli produce two different virulence factors including a polyglutamate capsule and a 3 components exotoxin. The capsule resists phagocytosis while the toxin is capable of injuring and killing the cells, which it binds to. Macrophage killing by the toxin is important in the spread of the disease. The infection is described as a toxigenic one with most of its pathogenesis relating directly to the toxin or to the toxin's influence on potentially harmful host mediators. Therefore, animal models based on the toxin alone are capable of simulating many of the key pathogenic events associated with the infection itself. This is important because the toxin can be manipulated far more safely than the bacteria itself. All small animal models to date using toxin challenge, have employed a single rapid bolus. Death in these models is relatively rapid extending from 1 to 3 hours after challenge depending on the dose of toxin. However, such a challenge is not consistent with the natural course of this infection which likely includes a gradual increase in the amount of toxin the host is dealing with. Such increases are reflected in the changes in blood bacteria concentrations that have been observed over time. Thus, an animal model simulating this progressive increase in toxin would better simulate conditions encountered clinically. This in turn would provide a more accurate assessment of evolving pathogenic events associated with toxin and, more importantly, would provide a better model to test the influence of therapies directed at inhibiting the toxin or its effects. The research underway for this project has so far shown that anthrax toxin administered as a 24 h infusion in Sprague-Dawley rats produces a prolonged and significantly different time course in lethality compared to a similar weight based bolus dose. This prolonged time course has permitted a more accurate assessment of the cardiopulmonary injury and cellular host response occurring in the model. In marked contrast to similarly lethal endotoxin models of sepsis, serum nitrate and nitrite levels were not increased either early (6h) or late (24h) following anthrax \toxin. Cytokine panels are now under analysis to better define the host inflammatory response to lethal anthrax toxin challenge in the model. In contrast to the unique sensitivity to anthrax toxin that Fischer rats are described as having in the literature, in controlled experiments we have now completed, Sprague-Dawley rats show the same pattern of response to toxin as Fischer animals, making our model even more applicable for laboratories elsewhere. Given the central role the macrophage is believed to have in the pathogenesis of anthrax infection, as part of this project experiments are now underway to investigate the influence of granulocyte-macrophage colony stimulating factor treatment during prolonged toxin infusion.